On-chip voltage regulator using feedback on process/product parameters

ABSTRACT

The present invention optimizes the performance of integrated circuits by adjusting the circuit operating voltage using feedback on process/product parameters. To determine a desired value for the operating voltage of an integrated circuit, a preferred embodiment provides for on-wafer probing of one or more reference circuit structures to measure at least one electrical or operational parameter of the one or more reference circuit structures; determining an adjusted value for the operating voltage based on the measured parameter; and establishing the adjusted value as the desired value for the operating voltage. The reference circuit structures may comprise process control monitor structures or structures in other integrated circuits fabricated in the same production run. In an alternative embodiment, the one or more parameters are directly measured from the integrated circuit whose operating voltage is being adjusted.

The present application is a divisional of U.S. Application Ser. No.10/628,711, filed Jul. 28, 2003, now U.S. Pat. No. 7,170,308 whichapplication is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to integrated circuits and inparticular to providing integrated circuits with optimized operatingvoltages.

Although the manufacture of integrated circuits is carefully controlled,inherent variations in the fabrication process cannot be avoided. Theseprocess-related variations translate into variations of functional andelectrical parameters of the manufactured devices and affect the deviceperformance. One example of a parameter that may be subject tovariations during the manufacturing process is temperature. Of course,there are numerous other process parameters that may vary, as well. Theresulting device parametric variations occur from lot to lot and fromwafer to wafer, but also within wafers and even within dice. They cancause variations in timing performance and operating margin of thefabricated integrated circuits.

Device parametric variations due to process variations can beconsiderable in magnitude and therefore have a critical impact on theyield of the fabrication process. Because of this, circuit designershave to accommodate these variations when designing the circuit.Specifically, they have to design the circuit so as to meet thespecification not only at optimal fabrication conditions but at processcorners. However, performance requirements are difficult to achieve atthe process corners. The designer thus has to weigh the goals of highperformance and high yield, forcing him to make a trade-off between thetwo goals.

SUMMARY OF THE INVENTION

The present invention uses the adjustment of the operating voltage ofintegrated circuits to optimize circuit performance and achieve higheryield per wafer. The adjustment is made based on one or more measuredproduct parameters affected by process variations.

In one embodiment, the present invention provides a method ofdetermining a desired value for an operating voltage of an integratedcircuit. The method comprises the steps of: on-wafer probing one or morereference circuit structures to measure at least one parameter of theone or more reference circuit structures; determining an adjusted valuefor the operating voltage based on the measured parameter; andestablishing the adjusted value as the desired value for the operatingvoltage of the integrated circuit. In this embodiment, the referencecircuit structures are structures distinct from the integrated circuitwhose operating voltage is being adjusted but fabricated in the sameproduction run. Process control monitor structures or other integratedcircuits fabricated on the same wafer or in the same lot as theintegrated circuit whose operating voltage is being adjusted may besuitably used as reference circuit structures.

In another embodiment, method comprises the steps of: measuring at leastone parameter of one or more circuit structures of an integratedcircuit; determining an adjusted value for the operating voltage basedon the measured parameter; and establishing the adjusted value as thedesired value for the operating voltage of that integrated circuit.

The parameter to be measured is preferably an electrical or functionalparameter of the integrated circuit or the probed reference circuitstructure, e.g., a leakage (stand-by) current or a circuit operatingspeed. Of course, more than one parameter may be measured and used foradjusting the operating voltage. Advantageously, any parametermeasurement is made while operating the examined circuit structures,whether reference structures or structures of the integrated circuititself, at a voltage having the nominal value.

In still other embodiments, the present invention provides methods ofproviding an operating voltage to an integrated circuit using a voltageregulator. The voltage regulator comprises: a voltage down-converterarranged to convert a chip-external supply voltage to a convertedvoltage based on a signal indicative of a desired value of the convertedvoltage, and output the converted voltage as the operating voltage; andan adjustable signal generator for adjustably generating the signalindicative of the desired value of the converted voltage. In theseembodiments, the signal generator is adjusted dependent on at least onemeasured electrical or operational parameter of one or more referencecircuit structures or at least one measured electrical or operationalparameter of one or more circuit structures of the integrated circuit.

In a preferred embodiment, the voltage regulator is an on-chipregulator.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention is described in more detail byway of example only, and not by way of limitation, in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an integrated circuit chipincorporating an on-chip voltage regulator, in accordance with preferredembodiments of the present invention;

FIG. 2 is a flow diagram of method steps for optimizing an outputvoltage delivered by the voltage regulator of FIG. 1; and

FIG. 3 is a schematic view of a wafer having built thereon a number ofintegrated circuit chips and process control monitor structures.

DETAILED DESCRIPTION OF THE INVENTION

With the trend in integrated circuit fabrication technology to reducedcharacteristic lengths, the operating voltage for an integrated circuitalso needs to be reduced. This is due primarily to the reduction inbreakdown voltage as circuit structures are more densely packed andtherefore distances between critical circuit structures are reduced.Voltage regulation becomes an important issue at reduced circuitoperating voltages. In order to provide operating voltages as low as,e.g., 3.3 V, 2.5 V, 1.8 V, 1.2 V or less, that are needed by modernintegrated circuits fabricated in sub-micron or nanometer technology, avoltage regulator is required that performs down-conversion from asupply voltage of typically 5 V or 12 V. To maintain tight regulationwith low fluctuation of the regulated voltage, the trend is to useon-chip voltage regulators, i.e., regulators integrated on the same chipas the integrated circuit.

FIG. 1 illustrates a semiconductor chip 10 having built thereon anintegrated circuit 12 and an on-chip voltage regulator 14. Achip-external power supply 16, which may be arranged on the same printedcircuit board (not shown) as the semiconductor chip 10, delivers asupply voltage Vext of, e.g., 5 V. The integrated circuit 12 requires anoperating voltage VCC lower than the supply voltage Vext. For example,it may be specified as requiring a nominal operating voltage of 3.3 V,2.5 V or 1.8 V. The voltage regulator 14 receives the supply voltageVext and outputs the voltage VCC, which is fed to the integrated circuit12. Specifically, the voltage regulator 14 includes a down-convertersection 18 and a signal generator section 20. The signal generatorsection 20 generates a signal representative of a target value of theoutput voltage VCC and supplies it to the down-converter section 18. Thedown-converter section 18 performs down-conversion of Vext and regulatesthe converted voltage to the target value as given by the signal fromthe signal generator section 20. The signal generator section 20 isadjustable or trimmable, so that the target value of VCC may be varied.Hence, by suitably adjusting the signal generator section 20, a desiredvalue of VCC can be obtained.

While only one integrated circuit 12 is illustrated in FIG. 1 for thesake of simplicity, a person versed in the art will easily appreciatethat two or more integrated circuits 12 may be fabricated on chip 10,which may all receive their operating voltage from voltage regulator 14.Each integrated circuit 12 can be any type of circuit, digital oranalogue. Its circuit technology (e.g., CMOS, bipolar or hybrid),fabrication technology and function are not critical to the invention.Possible realizations of the integrated circuit 12 comprise, but are notlimited to, a processor, a programmable logic device (PLD), anapplication-specific integrated circuit (ASIC), etc.

Due to variations in the manufacturing process, performance parametersof integrated circuits, such as, for example, operating speed, outputleakage current, and power consumption, may vary from chip to chip.These variations can be so large that some of the integrated circuitsmay, and typically do, fail to meet the specification, with the resultthat they have to be discarded. For example, higher leakage current Ioffof an integrated circuit generally implies higher circuit supply currentICC. In deep sub-micron chip fabrication technology, the leakage currentIoff may become very large, especially at process corners when allprocess-related variations are taken into account, leading to too high asupply current ICC.

Performance-characterizing parameters of an integrated circuit usuallydepend on the operating voltage of the circuit. Thus, varying thecircuit operating voltage is typically accompanied by concomitantvariations in one or more of these parameters. For example, lowering theoperating voltage typically lowers the leakage current of an integratedcircuit. On the other hand, increasing the operating voltage mayincrease the circuit operating speed.

The production yield in chip fabrication can be enhanced by adjustingthe operating voltage of integrated circuits based on measurements madeof electrical or operational parameters of (1) select reference circuitstructures fabricated in the same production run as the integratedcircuits or (2) the integrated circuits themselves. Specifically, thecircuit operating voltage is adjusted from a pre-set nominal value by anadjustment amount determined from the measured data. Suitably adjustingthe operating voltage can make integrated circuits acceptable whoseperformance parameters would otherwise have been outside thespecification. In this way, a significantly higher yield can beachieved. As an example, rough calculations have shown that on 90 nmtechnology up to about 10% of the total die area on a wafer can berecovered.

FIG. 2 illustrates steps to be taken in order to optimize the operatingvoltage for the integrated circuit 12 shown in FIG. 1. In step S1, dataare acquired by measuring one or more performance-characterizingparameters of one or more circuit structures. As indicated above, thesecircuit structures can be reference circuit structures fabricatedpreferably in the same lot or on the same wafer as the integratedcircuit 12. It is equally possible to obtain performance-characterizingdata from direct measurements of the integrated circuit 12.Advantageously, the one or more parameters are measured under nominaloperating conditions of the probed circuit structures. Specifically, theparameters are measured while the nominal operating voltage as specifiedby the designer is applied to the probed circuit structures.

Following step S1, an adjusted target value for the operating voltage ofthe integrated circuit 12 is determined in step S2 based on the acquiredmeasurement data. For example, in a case where the integrated circuit 12is designed for a nominal operating voltage of 1.8 V, an optimized valuefor the operating voltage may be, e.g., 1.7 V if the measured dataindicate that the leakage current Ioff of the integrated circuit 12 is,or is likely to be, too high at the nominal operating voltage. On theother hand, increasing the operating voltage to, e.g., 1.9 V maycompensate for unacceptable slowness of the circuit operating speed atthe nominal operating voltage. Evaluation of the measurement data anddetermination of the adjusted target value for the operating voltage maybe made based on empiric information or using mathematical algorithms orformulas. Of course, if the evaluation of the measured data reveals thatthe examined parameters are in fact within acceptable limits, noadjusted target value for the operating voltage is determined. Rather,the nominal value as given in the device specification is established asthe target value for the operating voltage.

Finally, in step S3, the voltage regulator 14 is set so as to deliverthe adjusted operating voltage. Specifically, the signal generator 20 isset so that the signal delivered by the signal generator section 20 tothe down-converter section 18 is indicative of the adjusted target valuefor the output voltage VCC as determined in step S2.

So-called process control monitor (PCM) structures are one advantageousexample of reference circuit structures suitable for being probed forperformance-characterizing parameters. Conventionally, when fabricatinga wafer, a set of test structures, e.g., individual transistors, diodesor other circuit elements, is fabricated on the wafer in addition to theintegrated circuits proper. These test structures are known as the PCMstructures. They may be implemented as separate cells on extra waferarea or integrated side by side with the integrated circuits on the samedie area. The PCM structures are strategically distributed across thewafer so as to deliver representative data for all areas of the wafer.After fabrication of the wafer, tests for operational and electricalparameters (also referred to as Process Control Monitor or E-Test) arecarried out on the PCM structures using suitable test equipment.Measurement data originating from this parametric testing can be used torefine the manufacturing process. For the purpose of illustration only,FIG. 3 schematically depicts a wafer 110 having built thereon a numberof integrated circuits 120. PCM structures 130 are formed on the wafer110 outside the die area of the integrated circuits 120. After the wafer110 is diced into chips, the PCM structures 130 are disposed of aswaste.

Rather than relying on data obtained from testing PCM structures, theadjustment of the operating voltage of a particular integrated circuitmay be based on data gained from parametric measurements of one or moreselected other integrated circuits fabricated in the same production runor on the same wafer as the integrated circuit whose operating voltageis being adjusted. Preferably, several integrated circuits are selectedon the same wafer. By strategically choosing several integrated circuitsout of the totality of integrated circuits 120, representativeperformance-characterizing data can be gained for all wafer areas. Thus,the selected integrated circuits function as reference circuits insubstantially the same way as the PCM structures. The parametermeasurements can be carried out before or after cutting the wafer 110into chips.

Alternatively, measurements for performance-characterizing parametersmay be made on a portion of the integrated circuit whose operatingvoltage is being adjusted. In this case, adjustment of the operatingvoltage of a specific integrated circuit may be based solely on the datameasured for that integrated circuit.

Measurement step S1, determination step S2 and setting step S3 of FIG. 2may all be carried out by the chip manufacturer as part of the variousfunctional and other test procedures typically performed on chips beforeshipping them. In particular, adjustments to the operating voltage ofindividual integrated circuits may be made prior to shipping based onmeasured parameters of reference circuit structures in PCM structure orother integrated circuits or on measured parameters of the individualintegrated circuit.

Alternatively, integrated circuit chips may have integrated thereon asuitable on-chip device that performs steps S1-S3 without the use ofadditional external equipment. This permits parameter measurements andoperating voltage adjustments even during use of the chips. For a betterunderstanding of such on-chip measurement and adjustment facility, referagain to FIG. 1 where a detection section 22 and an evaluation section24 are shown as part of chip 10. As the detection section 22 and theevaluation section 24 are optional features, they are depicted in brokenlines in FIG. 1. The detection section 22 is arranged to measure one ormore electrical or operational parameters of the integrated circuit 12.For example, detection section 22 may measure a voltage drop across aspecific structure or it may measure a leakage current or a circuitoperating speed. Detection section 22 delivers its measured signals tothe evaluation section 24. Evaluation section 24 is arranged todetermine, based on the measured signals, a desired value for theoperating voltage VCC of the integrated circuit 12 and to adjust, ifnecessary, the signal generator section 20 so that the latter suppliesto the down-converter section 18 a signal indicative of the desiredvalue as determined by the evaluation section 24. Advantageously, thedetection section 22 may be arranged to take parameter measurementsrepeatedly during operation of the integrated circuit 12, e.g.,continuously or in regular intervals.

The evaluation section 24 may be implemented using, for example,programmable logic or a processor. A programmable logic device can beeasily programmed to perform the functions of the evaluation section 24.Similarly, a programmable logic device may also be used to form thesignal generator section 20. Programmable or trimmable elements, e.g.,fuses, in the signal generator section 20 enable easy adjustment of thetarget value of the operating voltage VCC.

In an alternative embodiment, detection section 22 and evaluationsection 24 may be arranged separately from chip 10, yet on the sameprinted circuit board.

In summary, the present invention permits the performance of integratedcircuits to be optimized and the yield to be increased by adjusting thecircuit operating voltage using feedback on process/product parameters.While preferred embodiments have been described above, it will beapparent to those skilled in the art that modifications can be madewithout departing from the spirit and scope of the invention.

1. A method of determining a desired value for an operating voltage ofan integrated circuit, said method being performed on a same chip as theintegrated circuit and comprising the steps of: probing one or morereference circuit structures on the chip to measure at least oneparameter of said one or more reference circuit structures whileoperating said one or more reference circuit structures at a voltagehaving a pre-set nominal value; determining on the chip an adjustedvalue for said operating voltage based on said measured parameter; andestablishing on the chip said adjusted value as said desired value forsaid operating voltage by converting a voltage received from an off-chipsource to said adjusted value.
 2. The method of claim 1, wherein saidparameter is an electrical or operational parameter of said one or morereference circuit structures.
 3. The method of claim 2, wherein saidparameter is a leakage current or an operating speed of said one or morereference circuit structures.
 4. The method of claim 1, wherein saidprobing step comprises measuring a plurality of different parameters ofsaid one or more reference circuit structures, and said determining stepcomprises determining an adjusted value based on said measured pluralityof different parameters.
 5. The method of claim 1, wherein said one ormore reference circuit structures comprise one or more process controlmonitor structures.
 6. A method of determining a desired value for anoperating voltage of an integrated circuit, said method being performedon a same chip as the integrated circuit and comprising the steps of:measuring on the chip at least one parameter of one or more circuitstructures of said integrated circuit while operating said one or morecircuit structures at a voltage having a pre-set nominal value;determining on the chip an adjusted value for said operating voltagebased on said measured parameter; and establishing on the chip saidadjusted value as said desired value for said operating voltage byconverting a voltage received from an off-chip source to said adjustedvalue.
 7. The method of claim 6, wherein said parameter is an electricalor operational parameter of said one or more circuit structures of saidintegrated circuit.
 8. The method of claim 7, wherein said parameter isa leakage current or an operating speed of said one or more circuitstructures of said integrated circuit.
 9. The method of claim 6, whereinsaid measuring step comprises measuring a plurality of differentparameters of said one or more circuit structures of said integratedcircuit, and said determining step comprises determining an adjustedvalue based on said measured plurality of different parameters.
 10. Themethod of claim 6, wherein said parameter is repeatedly measured duringnormal operation of said integrated circuit.
 11. The method of claim 6,wherein said parameter is measured during testing of said integratedcircuit.
 12. A circuit for determining a desired value for an operatingvoltage of an integrated circuit, comprising: means formed on a samechip as the integrated circuit for measuring at least one parameter ofone or more circuit structures of said integrated circuit whileoperating said one or more circuit structures at a voltage having apre-set nominal value; means formed on the same chip for determining anadjusted value for said operating voltage based on said measuredparameter; and means formed on the same chip for establishing saidadjusted value as said desired value for said operating voltage byconverting a voltage received from an off-chip source to said adjustedvalue.
 13. The circuit of claim 12 wherein said parameter is anelectrical or operational parameter of said one or more circuitstructures of said integrated circuit.
 14. The circuit of claim 12wherein said parameter is a leakage current or an operating speed ofsaid one or more circuit structures of said integrated circuit.
 15. Thecircuit of claim 12 wherein said parameter is measured while operatingsaid one or more circuit structures of said integrated circuit at avoltage having a pre-set nominal value.